The present invention relates to hermetic compressors and more particularly to two stage compressors using carbon dioxide as the working fluid.
Conventionally, multi-stage compressors are ones in which the compression of the refrigerant fluid from a low, suction pressure to a high, discharge pressure is accomplished in more than one compression process. The types of refrigerant generally used in refrigeration and air conditioning equipment include clorofluorocarbons (CFCs) and hydrochlorofluorocarbon (HCFC). Additionally, carbon dioxide may be used as the working fluid in refrigeration and air conditioning systems. By using carbon dioxide refrigerant, ozone depletion and global warming are nearly eliminated. Further, carbon dioxide is non-toxic, non-flammable, and has better heat transfer properties than CFCs and HCFC, for example. The cost of carbon dioxide is significantly lower than CFC and HCFC. Additionally, it is not necessary to recover or recycle carbon dioxide which contributes to significant savings in training and equipment.
In a two stage compressor, the suction pressure gas is first compressed to an intermediate pressure. The intermediate pressure gas can be directed to the second stage suction side or cooled in the unit heat exchanger before delivery to the second stage suction. The intermediate pressure gas is next drawn into a second compressor mechanism where it is compressed to a higher, discharge pressure for use in the remainder of a refrigeration system.
The compression mechanisms of the two stage compressor may be stacked atop one another on one side of the motor, or positioned with one located on each side of the motor. When the compression mechanisms are located on opposite sides of the motor, each compression mechanism is provided with an oil sump which provides lubricating oil to the respective compressor components. Oil in the lower, main sump provides lubrication to the first compression mechanism and is drawn through a passage in the drive shaft to lubricate the second compression mechanism. Oil from the upper sump also provides lubrication to the second compression mechanism.
Problems may occur if the excess oil does not return to the main oil sump during compressor operation and collects in the upper sump. Such problems include overfilling of the upper sump and depleting the supply of oil in the lower, main sump. If the amount of oil in the lower sump is reduced, the amount of oil required to lubricate the bearing surfaces may be insufficient.
During shutdown of the compressor, a portion of the unused or excess oil may return to the main oil sump by gravity. The amount of oil between bearing surfaces is significantly reduced or eliminated. When the compressor is restarted, the bearings surfaces come into contact with one another which can damage these surfaces.
It is desired to provide a two stage hermetic compressor which uses carbon dioxide as the working fluid and is provided with a lubrication system that improves lubrication during startup and operation of the compressor.